md-option has black text on dark background in dark themes

[wulfsberg]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

md-options in an md-select are shown with a readable light text when shown on dark background

What is the current behavior?

md-options always have black text

What are the steps to reproduce?

Plunker here:
http://plnkr.co/edit/UmStZu0puH1O7I2T78nd?p=preview

What is the use-case or motivation for changing an existing behavior?

The black text is barely readable against the dark background of a dark theme.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Material2-beta.5

Is there anything else we should know?

[angular-automatic-lock-bot]

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